(1) Field of the Invention
The present invention relates to an image forming device that forms an image on a recording sheet having an unfixed image transferred thereon by thermally fixing the unfixed image, and relates to an image forming method executed in the image forming device.
(2) Description of the Related Art
In this kind of image forming device, when a recording sheet is carried to a fixer, a temperature of a fixing roller for heat-fixing an image is controlled. When a recording sheet is not carried to the fixer, the image forming device stands by, stopping rotation of the roller for noise abatement, energy saving and such, and controlling the temperature of the roller so as to make quick response in a case of receiving an emergent execution instruction for image formation.
Hereinafter, a preset temperature of a roller at heat-fixing is referred to as a fixing temperature, and a preset temperature of a roller on standby is referred to as a standby temperature.
FIG. 16 is a view to show a temperature transition of a roller in a conventional image forming device, and show an example when both the fixing temperature and the standby temperature indicate 180° C. Note that the bold solid line indicates a preset temperature set for the temperature control, and the thin solid line indicates a detected temperature of a circumferential surface of the roller.
As shown in FIG. 16, if the roller is stopped immediately after the completion of the fixing process (point a in FIG. 16), an amount of heat discharge is decreased and the detected temperature of the roller rises sharply (point b in FIG. 16).
The sharp rise in the detected temperature causes excessively high temperature in the roller, which may result in deterioration of members in the vicinity of the roller. Accordingly, this sharp rise in the temperature needs to be suppressed as much as possible. Hereinafter, the rise in the temperature due to this sharp rise is occasionally referred to as overshoot.
In response to the above, a conventional technique is proposed as follows. After the fixing process is completed, and before the roller is stopped, the roller is rotated without being heated for a certain period of time for heat release. After the temperature of the roller is decreased to some extent, the roller is stopped (See Japanese Unexamined Patent Application Publication H6-202526, Japanese Unexamined Patent Application Publication H11-249489).
FIG. 17 is a view to show temperature transition of the roller on the assumption that the above conventional technique is applied, and shows an example in which both the fixing temperature and the standby temperature are 180° C., and in which the temperature is once decreased to 155° C. after the fixing process.
As shown in FIG. 17, after the completion of the fixing process, the preset temperature is decreased to 155° C. with the roller kept rotating (point a in FIG. 17). Following this, the rotation of the roller is stopped, and the operation is switched to be in the standby state (point b in FIG. 17), and the temperature is controlled to be 180° C. which is the standby temperature. Then, the detected temperature rises sharply. However, compared with the case in which the control is not performed (FIG. 16), the peak temperature (point c in FIG. 17) of the increasing temperature can be kept low.
However, the fixing temperature is usually changed according to an environmental temperature, a type of a recording sheet and such. The aforementioned control is effective when the fixing temperature is set as high as or higher than the standby temperature. However, the fixing temperature is set lower than the standby temperature and accordingly there is a large gap between the fixing temperature and the preset temperature, which presents new problems of the overshoot.
The following is a detailed explanation of the problems.
FIG. 18 shows an example in which the fixing temperature is 145° C. and the standby temperature is 190° C.
In this case, as shown in FIG. 18, after the completion of the fixing process, the preset temperature is decreased to 120° C. with the roller kept rotating (point a in FIG. 18). Following this, the preset temperature is increased to 190° C. and the temperature control is started (point b in FIG. 18). However, since the preset temperature is significantly increased from 120° C. to 190° C. at the point b in FIG. 18, the detected temperature overshoots to a large extent (point c in FIG. 18).
This overshoot causes excessively high temperature of the roller, which may deteriorates members in the vicinity of the roller. Accordingly, it is necessary to suppress this overshoot as much as possible.
In addition, if the control shown in FIG. 18 is not performed, as shown in FIG. 19, when the preset temperature (145° C.) for fixing is much lower than the preset temperature (190° C.) in the standby state, the following occurs. If the rotation of the roller is stopped immediately after the completion of the fixing process (point a in FIG. 19), the temperature of the roller rises, and the image forming device is switched to be in a standby state and controls the temperature of the roller to be 190° C., which results in heating the roller. Thus, the detected temperature overshoots to a large extent (point b in FIG. 19).